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转圈过程（边界处理）遵循循环不变量的原则，坚持一个原则处理每一条边，左闭右开处理

class Solution {
public:vector<vector<int>> generateMatrix(int n) {vector<vector<int>> num(n, vector<int>(n, 0));int startx = 0;int starty = 0;int offset = 1;int count = 1;int loop = n/2;int i;int j;while(loop --){for( j = starty; j < n - offset; j++){num[startx][j] = count;count ++;}for( i = startx; i < n - offset; i++){num[i][j] = count;count ++;}for( ; j > starty; j--){num[i][j] = count;count ++;}for( ; i > startx; i--){num[i][j] = count;count ++;}startx++;starty++;offset++;}if(n % 2){num[n/2][n/2] = count;}return num;}
};